1. Field of the Invention
This invention relates to a magnetic tape cassette used for recording and reproducing sounds or the like, and more particularly to a magnetic tape cassette in which relative positional slippage between a magnetic tage and a tape recorder using said tape is minimized.
2. Description of the Prior Art
Discs (so-called records) and magnetic tapes are commonly used for recording and/or reproducing sounds or the like. The magnetic tapes are widley used for both recording and reproduction since information recorded on a magnetic tape can easily be erased for reuse, whereas the discs in general are used only for reproduction. Among the magnetic tapes, a so-called compact magnetic tape cassette has come into wide use because of its handiness and compactness.
When a magnetic tape is used for recording sounds, the sounds are generally converted to analog signals which are then recorded on the magnetic tape. During reproduction, on the other hand, said signals are read out from the magnetic tape and reproduced into sounds by use of an amplifier and a speaker connected thereto.
Recently, however, digital signals which are not easily affected by noises and which have a high signal-to-noise ratio have become to be used as substitute for analog signals to improve sound quality in transmission, recording and reproduction. Better sound quality is obtained by use of the digital signals since signal-to-noise ratio can be improved by processing the digital signal. Morever, in the case as in the transmission through a broadcating satellite wherein analog signals are likely to be affected by noises, noiseless high fidelity sounds can be transmitted by use of digital signals since errors can be corrected in the digital signals. Accordingly, the use of digital signals in the sound transmission field is widely spreading. In case that digital signals are used in sound transmittion, means for recording and reproducing said signals would also be needed. Even in this case, discs, magnetic tapes and the like are commonly used, and, among others, the magnetic tape cassette is preferably used because of its compactness and handiness as is in analog signal recording and reproduction.
A recording area which covers a frequency band on the order of 100 KHz is necessary in digital signal recording, whereas of frequency band on the order of 100 KHz is sufficient for analog signal recording. That is, in case where a magnetic tape is used in sound recording by means of digital signals, which case is commonly called DAT (Ditital Audio Tape) recording process, a frequency band on the order of 100 KHz should be recorded on the magnetic tape on which a frequency band on the order of 10 KHz has conventionally been recorded.
Therefore, in order to expand the available frequency band, a helical rotary head recording system in which the recording and reproducing head is rotated, and a multitrack fixed head recording system in which an increased number of recording and reproducing tracks are employed, have come into use. Since the helical rotarty head is of the same type as that used for VTR's of which recording capacity may extend to a band on the order of 5 MHz, there exists substantially no problem with respect to recording of a frequency band on the order of 100 KHz. In case of the multitrack fixed head recording system, about then tracks are needed for the DAT recording of a frequency band on the order of 100 KHz since the recording capacity of one track thereof is restricted to a frequency band in the range of about 10-15 KHz due to limitation in the recorded wavelength. Further, when a dynamic range and error correction are taken account of, the number of tracks needed would increase up to 14-20. For the double-side recording, the number of the tracks would generally be doubled. In practice, the use of 36 tracks (18 tracks for each side) has been proposed. When a conventional compact tape cassette is used in a 36-track fixed head recorder, 36 tracks would have to exist in a tape width of 3.81 mm. Thus, the width of one track would become only about 100 .mu.m. In the DAT, the relative positional relationship between a head and a tape becomes very critical since the tape width for one track is as narrow as about 100 .mu.m; whereas in the conventional analog recording, it is not necessary to maintain said relationship due to large tape width for one track, for monaural recording uses only two tracks (one track for each side) and stereo recording uses only four tracks (two tracks for each side).
The slippage between a head and a tape mainly results from change in the tape path due to swinging of tape wind-up cores and/or rotatable guide rollers which occurs in a gap between the tape wind-up cross and a cassette casing as well as between rotatable guide rollers and the cassette casing. Particularly, when the tap feeding speed is changed, e.g., from the normal play back to the fast forwarding, the change in tension of the tape would affect the positions of the wind-up cores and/or the rotatable guide rollers, so that the tape path is likely to become unstable.
Thus, to prevent the slippage between the head and the tape, it is necessary to reduce the influence of the tape tension change at the feeding speed change by eliminating the swinging of the tape wind-up cores and/or the rotatable guide rollers or by providing more or less back tension to the tape when the tape is used. Accordingly, in general, a pair of friction sheets are loosely disposed on and under the tape wind-up cores to limit the fluctuation of the tape and to provide back tension thereto. In a conventional magnetic tape cassette, however, the friction sheets are merely disposed in the lower and upper sides of the tape wind-up cores. That is, the top and the bottom of each post of the fixed guide rollers and of the fixed guide pins are not covered. Accordingly, the tape path becomes so unstable that the tape cannot be uniformly wound up on a tape wind-up core. Thus, when the opposite side of the cassette is used, the delivery point of the tape becomes too unstable to fix the tape path, which results in undersirable slippage between the tape and the magnetic head.